A PROCESS FOR PRODUCTION OF CERAMIC POWDERS BY SPRAY DRYING

Abstract: A process for spray drying of ceramic suspensions for the production of a powder which is dust free, sufficiently stable for storage and with good flow characteristics. The ceramic suspension is spray dried in a chamber (1) to a powder with a water content which is 0.1-10 % higher than required in the final powder and hereafter fluidized in a fluid bed (6) with ambient air for an evaporative cooling process, whereby the water content is reduced to the required value at the same time as the powder is cooled. The produced powder has a high water content without tendency to lumping.

Description: A process for production of ceramic powders by spray drying The present invention relates to a process for producing powdered ceramic products by spray drying an aqueous suspension, including dispersants and colorants. Examples of products produced according to this process are oxides and silicates and similar of Aluminium, Magne- sium as well as other metals, such as clay, ferrites, kaolin and steatite. The powdered products are used within various fields such as filler and coating of paper, ceramics, catalyst carriers etc. A well-known industrial process to dry suspensions of ceramic products is one-stage spray drying, cf. Masters, K., Spray Drying Handbook, 1991 p. 569 ff. By this pro¬ cess a kaolin powder with 0.5-3.0% residual free moisture is obtained by using drying air inlet temperatures of e.g. 400-600°C and outlet temperatures of 95-120°C (p. 504). The emphasis in prior art has been to reduce total energy consumption by scrubbing and/or heat exchanging the sus¬ pension with the spent drying gas, which has a relatively high temperature. This forms a part of a number of prior art patents. US Patent Specification No. 4.642.904 describes a system in which a clay slurry is dewatered by scrubbing and then preheated by heat exchangers before it is spray dried in one stage to a powder with about 1 % residual free moisture. US Patent Specification No. 4.717.559 describes a method in which calciner waste heat is used for supplying heat to the suspension before a one-stage spray drying process. WO 92/14530 describes a process in which a feed of kaolin slurry passes a heat exchanger and a flash concentrator before being spray dried in a one-stage spray drying process. Known processes for spray drying of ceramic suspensions are one-stage drying processes. The produced powders are suitable for storage, however, they are often very dusty due to a fine particle size and low free moisture content and they have poor flow properties. With a view of avoiding these drawbacks it has been suggested to produce dust free and free flowing ceramic powders with large particle size and increased free moisture content in a one-stage spray drying process. This is possible by operating at decreased outlet tempera- tures, a process well known to a person skilled in the art. By doing so, however, powders will inevitably form unacceptable lumps during storage. It has now turned out that ceramic suspensions, as defined above, can be spray dried into a powder with the above mentioned good properties using a process characterized in that: (a) in a 1st stage the suspension is spray dried to a powder (b) in a 2nd stage the powder is cooled evaporative with ambient air. The powder produced according to the invention is dust free, stable during storage, it has good flow properties, and has a uniform and high bulk density as well as good dispersing properties. Suspensions dried as per this pro¬ cess are exposed to a minimum of heat treatment, which is important for suspensions containing organic components such as dispersants and brighteners. The process is specially appropriate for spray drying of products which, due to a small primary particle size, are not easily spray dried into a dust free, free-flowing, high quality powder suitable for storage. Furthermore, the process opens possibilities of savings in operation as well as investment costs. By using this procedure the powder is cooled as a conse- quence of evaporation of moisture in the powder. The energy required for the evaporation equals the energy released when the powder is cooled. In the well-known two-stage drying processes, as per above p. 587, energy must be provided by means of hot air to the first part of the fluid bed, and the cooling is performed by cold air. The process according to the invention makes it possible to dry suspensions for the production of powders with properties which have hitherto been regarded non-obtain- able e.g. storable, homogeneous, free flowing and dust less kaolin powder with a relatively high water content. It leads to an improved heat economy for a given product quality in relation to what is considered possible by using the prior art as mentioned in above Masters p. 569. By using a special procedure according to the present invention the atomization of the suspension is performed by means of a centrifugal atomizer with continuously vari¬ able revolutions per minute. It is then possible to ad- just the particle size of the final powder as well as to obtain a technical effect in form of a continuous adjust- ment and control of the water content in the powder led into the fluid bed with higher accuracy than has hitherto been possible, based on measurements of the outlet tempe¬ rature of the drying air. In a preferred embodiment according to the invention, peripheral velocity of the centrifugal atomizer wheel of between 50 and 90 m/s is used. As a result it is possible to produce large, free-flowing particles. The following is a detailed description of the invention with reference to the drawing, which is a schematic re¬ presentation of an apparatus suitable for performing the procedure, consisting of a spray drying chamber 1 with a fluidized bed 2. The suspension is added through the pipe 3 and is atomized by the centrifugal atomizer 4 into the chamber 1, where by means of hot air which is added through duct 5, the drop- lets are dried to a water content which is 0 to 10 % higher than required in the final powder. The powder falls down into the fluidized bed 6 and is fluidized by means of ambient air added through duct 7 and the perforated plate 8. An evaporation to the required water content takes place while the powder is cooled. The final powder leaves the fluid bed at 9 and can be led to storage (not illustrated). Drying air from the spray dry¬ ing chamber and from the fluid bed is led through the ducts 10 and 11 to duct 12. Fine particles are separated in the cyclone 13 and led to storage via the outlet (not illustrated). It is to be noted that the procedure according to the invention is not limited to be performed in a two-stage apparatus as illustrated, but can be performed in other combinations of a spray drying chamber with equipment for pneumatic evaporative cooling. The process of the present invention will be further ex- plained by comparing the following two examples, whereof Example 1 describes the beforementioned prior art and Example 2 describes the present invention. Example 1: was performed in a one-stage spray drying plant as de¬ picted in the above mentioned Masters 5. ed fig 4.10a. The spray drying chamber had a cylindrical diameter of 12.5 m and a total height of 23 m. 50.000 kg/h of a sus¬ pension of kaolin with 55 % total solids and a temperature of 60°C was fed to a centrifugal atomizer with wheel dia¬ meter 400 mm, rotating at 3500 rpm. The drying air inlet/- outlet temperatures were 500/95°C. The produced, dry kao¬ lin powder had a residual free moisture content of 0.5 %. According to internal standards it was very dusty with poor flow properties; it did not form lumps during storage. Example 2: is performed according to the present invention in two stages. The same spray dryer as in example 1 performs the 1st drying stage. From this dryer the powder falls into a 2 fluid bed with 8 m sieve plate area where it is fluidized with unheated ambient air at 25°C. The spray dryer air inlet/outlet temperatures are 300/70°C. Kaolin powder from the spray dryer chamber has 7 % free moisture, which is reduced to 5 % during the evaporative cooling in the fluid bed. The powder is non-dusting with good flow properties. It does not form lumps during storage.

Claims: P a t e n t C l a i m s: 1. A process for producing powdered ceramic products by spray drying an aqueous suspension, including dispersants ' and colorants, c h a r a c t e r i z e d in that (a) in a 1st stage the suspension is spray dried to a powder (b) in a 2nd stage the powder is cooled evaporative with ambient air. 2. A process according to claim 1, c h a r a c t e r ¬ i z e d in that the temperature of the outlet air from the 1st stage is controlled within the range of 50 to 100°C. 3. A process according to claim 1, c h a r a c t e r - i z e d in that the temperature of the outlet air from the 1st stage is controlled to obtain a free moisture content in the range of 0.5 to 30 % in the powder leaving the 1st stage. 4. A process according to claim 1, c h a r a c t e r ¬ i z e d in that the free moisture in the powder from the 2nd stage is in the range of 0 to 30 %. 5. A process according to claim 1, c h a r a c t e r - i z e d in that the free moisture content in the powder from the 1st stage is 0 to 10 % higher than the free moisture content in the powder from the 2nd stage. 6. An apparatus for producing powdered ceramic products by spray drying of an aqueous suspension, including dispersants and colorants, c h a r a c t e r i z e d in that it comprises a spray dryer wherein a 1st drying stage is carried out and an evaporative cooler adapted to supply ambient air directly into the powder in a 2nd stage. 7. An apparatus according to claim 6, c h a r a c t e r ¬ i z e d in that the evaporative cooler is a fluid bed. 8. An apparatus according to claim 6, c h a r a c t e r ¬ i z e d in that the spray dryer comprises a centrifugal atomizer with continuously variable revolutions of the atomizer wheel. 9. An apparatus according to claim 6, c h a r a c t e r ¬ i z e d in that the peripheral velocity of the centrifugal atomizer wheel is continuously variable in the range of 50 to 90 m/s. 10. Use of a fluid bed for the production of a powdered ceramic product in a 2nd stage after spray drying an aqueous suspension, including dispersants and colorants

Inventors: FREDSTED SOEREN, GETLER JENS, GIBSON STUART, NIELSEN PER, FREDSTED, SOEREN, GETLER, JENS, GIBSON, STUART, NIELSEN, PER
Applicants: ANHYDRO AS, FREDSTED SOEREN, GETLER JENS, GIBSON STUART, NIELSEN PER, APV ANHYDRO A/S, FREDSTED, SOEREN, GETLER, JENS, GIBSON, STUART, NIELSEN, PER
Application year: 1994
Main classification: C04B35
IPC codes: C04B35, F26B3